The present disclosure relates to a technical field of displays, and more particularly to a pixel driving circuit and a display device.
Organic light emitting diode (OLED) display devices have advantages of being self-luminous, having low driving voltages, high luminous efficiency, short response time, high sharpness and contrast, nearly 180° viewing angles, and wide operating temperature ranges, and allowing implementation of flexible displays and large area full color displays, etc., and have been commonly recognized by industry as display devices with the most development potential.
According to driving methods, OLED display devices may be divided into two types: passive matrix OLEDs (PMOLEDs) and active matrix OLEDs (AMOLEDs), i.e., two types: direct addressing and thin film transistor (TFT) matrix addressing. AMOLEDs have pixels arranged in arrays, belong to active display types, have high luminous efficiency, and are usually used for large scale display devices of high sharpness.
AMOLEDs are current-driven devices. When currents flow through OLEDs, OLEDs emit light, and luminance of emitted light is determined by currents flowing through OLEDs themselves. Most existing integrated circuits (ICs) only transmit voltage signals. Therefore, pixel driving circuits of AMOLEDs need to complete tasks of converting voltage signals into current signals.
Conventional AMOLED pixel driving circuits are usually 2T1C, i.e., with structures including two TFTs and one capacitor. Referring to FIG. 1, an existing 2T1C pixel driving circuit includes a first TFT T10, a second TFT T20, a capacitor C10, and an OLED D10. The first TFT T10 has a gate electrically connected to a drain of the second TFT T20, a source receiving a positive power supply voltage OVDD, and a drain electrically connected to an anode of the OLED D10. The second TFT T20 has a gate receiving a gate driving signal Gate, a source receiving a data signal Data, and the drain electrically connected to the gate of the first TFT T10. The capacitor C10 has an end electrically connected to the gate of the first TFT T10, and the other end electrically connected to the source of the first TFT T10. The OLED D10 has the anode electrically connected to the drain of the first TFT T10, and a cathode receiving a negative power supply voltage OVSS. When the 2T1C AMOLED pixel driving circuit operates, a current flowing through the OLED D10 satisfies the following equation.I=k×(Vsg−Vth)2 
where I is the current flowing through the OLED D10, k is a constant coefficient related to characteristics of the first TFT T10, Vsg is a voltage difference between the source and the gate of the first TFT T10, and Vth is a threshold voltage of a driving TFT (i.e. first TFT T10). It can be seen that the current flowing through the OLED D10 is related to the threshold voltage of the driving TFT.
Due to reasons, such as unstable manufacturing processes, a threshold voltage of a driving TFT of each pixel driving circuit in panels is different. After TFTs are used for a long time, TFT material aging and varying occur, causing threshold voltages of driving TFTs to drift, resulting in a problem that currents flowing through OLEDs are unstable. Therefore, panels display non-uniformly. In conventional 2T1C circuits, threshold voltage drift of driving TFTs cannot be improved by adjustment. Therefore, new TFTs or new signals need to be added, in order to reduce impacts of threshold voltage drift. That is, AMOLED pixel driving circuits are caused to have compensation functions.